Surface acoustic waves (i.e., "SAWs"), also known as Rayleigh waves, have been known since the middle of the nineteenth century. However, it was not until much later that the phenomenon of SAW propagation was first exploited for its applications to electronic devices. Acoustic wave devices known in the art commonly consist of a substrate on which a conductive material is deposited in a predetermined pattern. The patterned conductive material is known as an interdigital transducer (i.e., an IDT). R. M. White et al., Appl. Phys. Let., Volume 7, Number 12, pages.314-316 (Dec. 15, 1965), describes the use of the IDT as an efficient technique for the generation and detection of surface acoustic waves on a piezoelectric surface. An IDT may be suitably connected to an electrical input so that the refractive index in a crystal is changed as required by acoustic-optic applications. See, e.g., K. S. Buritskii et al., Soy. Tech Phys. Lett. 17(8) pp. 563-565 (1991) and L. Kuhn et al., Appl. Phys. Lett. 17(6) pp. 265-267(1970). In other applications, an IDT on one end of a substrate surface may be connected to a source of the frequency waves (e.g., television antenna --radio frequency) and an IDT on the other end of the substrate surface may be Connected to a device designed to receive a predetermined frequency (e.g., radio frequency for a specific television channel). The design of the IDT (i.e., the pattern of the conductive materials on the surface of a particular type of substrate) determines how the frequency will be controlled (e.g., which channel is received).
The types of acoustic waves which may be generated in a given crystal depend upon the piezoelectric-elastic-dielectric (i.e., PED) matrix of the crystal, which in turn depends on the crystal structure. In other words, not all materials are suitable for SAW generation, and materials which are suitable for SAW generation may not be suitable for generation of other types of acoustic waves. The properties of the substrate (e.g., the crystal structure) will determine the type of acoustic wave that will be generated, mechanism of the control and how high a frequency can be controlled.
Radio frequency control devices using substrates capable of controlling the received radio frequency by the generation of SAWs are known in the art For example, R. S. Wagers et al., IEEE Transactions on Sonics and Ultrasonics, Vol SU-31, No. 3, pages 168-174 (May 1984) discloses SAW devices based on lithium niobate. In these SAW devices the SAWs, generated by an IDT connected to a source of radio frequency waves, propagate through a y-cut lithium niobate crystal at a rate of about 3500 meters per second. This permits these SAW devices to be useful as radio frequency controllers in, for example, conventional television.
Acoustic waves, other than SAWs, may be generated in bulk crystal. For example, the Bleustein-Gulyaev wave (i.e., B-G wave) has been both mathematically postulated and experimentally proven to exist in crystals having 6 mm or mm 2 crystal symmetries, (see e.g., J. L. Bleustein, Appl. Phys. Lett., volume 13, Number 12, Pages 412-413 (Dec. 15, 1968), and C. -C. Tseng, Appl. Phys. Lett. Volume 16, Number 6, Pages 253-255 (Mar. 15, 1970)); and surface skimming bulk waves (i.e., SSBWs) have been shown to propagate on the surface of the crystal and to gradually propagate partially into the depths of the crystal. Such waves (both SSBWs and B-G waves) generally propagate faster than conventional surface acoustic waves. SSBWs have been generated in lithium tantalate and lithium niobate at a rate of about 4100 meters per second and about 5100 meters per second, respectively (see Meirion Lewis et al., 1977 Ultrasonics Symposium Proceedings IEEE Cat #77CH1264-1SU pages 744-752). B-G waves have been in Bi.sub.12 GeO.sub.29 (i.e., "BGO") and Ba2NaNb.sub.5 O.sub.15 (i.e., "BNN") to possess velocities of 1694 m/sec and 3627 m/sec, respectively (see C. -C. Tseng, Appl. Phys. Lett. Volume 16, Number 6, Pages 253-255 (Mar. 15, 1970)).
Since potassium titanyl phosphate (i.e., KTP) crystals are widely known to have a high nonlinear optical coefficients and resistance to optical damage, the SAW properties of rubidium exchanged KTP have been investigated relative to use in acousto-optic devices. K. S. Buritskii et al., Electronics Letters, Vol. 27, No. 21, pages 1896-1897 (Oct. 10, 1991), discusses the excitation of SAWs in Rb:KTP (i.e., a slab waveguide formed by Rb ion exchange on the surface of a single crystal of KTP). The velocity of the SAW generated in this waveguide was about 3900 meters per second. Buritskii et al., Sov. Tech. Phys. Lett., Volume 17, Number 8, pages 563-565 (August 1991) discusses the fabrication of a planar acousto-optic modulator using a Rb:KTP waveguide.
Acoustic frequency mixing devices involve a solid substrate to which at least two input signals are applied. In one type of device, convolvers, the input signals are applied using separate input IDTs in a manner which allows the acoustic waves to propagate towards each other and convolve to generate an output. In another type of device, correlators, the input signals are applied in a manner which allows the waves to propagate in the same direction and correlate to generate an output. Convolvers and correlators have various uses. In wireless communication such as cellular phones, for example, an important consideration is the reduction of crosstalk (interference); and one way to minimize the interference is to use nonlinear acoustic convolvers to code the signals so that interference is reduced. Radar system devices use correlators and convolvers to compare a signal under scrutiny with a local reference signal in the receiver.
The number of devices requiring frequency control has grown in number and complexity, and the demand for controlling higher frequencies, such as those needed for microwave generators and high definition television, has grown commensurately. The effectiveness of acoustic frequency mixing devices is also influenced by loss of power density in the device. For example, in wireless communications such as cellular phones, convolver effectiveness can be improved by increasing the nonlinearity and reducing signal losses. Similarly, devices in Radar systems such as convolvers and correlators could also be improved greatly if there were better nonliner acoustic materials or better ways to increase power density without the need to increase the total input power (J. H. Fischer, J. H. Cafarelta, D. R. Arsenault, G. T. Flynn, and C. A. Bouman, IEEE Proceedings, V 75, p 100-115, 1987). Acoustic waveguides therefore could be used to miniaturize the nonlinear SAW device to increase the power density without requiring higher power input.
Only a limited number of solid substrate materials have been identified as effective for use in acoustic frequency mixing devices. There is interest in identifying new effective materials for this application.